1. Field of the Invention
This invention relates generally to measurement methods and specifically to controllers for laser surgery.
2. Discussion of Prior Art
Malformed blood vessels just beneath the skin surface cause skin disorders such as birthmarks, spider veins, rosacea, and hemangenomas. These disorders respond to treatment by an intense pulse of laser radiation.
For example, a birthmark known as "Port Wine Stain" (PWS), named for its appearance, is one specific disorder that is treated with a laser radiation pulse. These blemishes appear at infancy as pink areas, usually on the face and neck, but darken to purplish red during childhood.
PWS occurs when an abnormally high concentration of blood vessels forms in the papillary and reticular layer of derma, just beneath the two outer skin layers, the statum corneum and stratum Malpighii, collectively identified with the epidermis. The epidermis is typically 0.1 millimeters thick in a typical laser treatment area, while the interlocking projections of the papillary dermis range from 0.5 to 0.9 millimeters thick.
Thus, the blood vessels targeted for treatment typically lie 0.6 to 1.0 millimeters below the skin surface in young facial skin.
A visible yellow wavelength, 585 nanometers, is the accepted optimum choice for the treatment laser. This wavelength traverses the epidermis with relatively little absorption, but is highly absorbed by the blood vessels. In effect, the outer skin layer acts like a slightly milky pane of glass with dark blood vessels pressed against its underside.
When laser treatment is applied to a predetermined skin site, the intense beam of laser light passes through the skin to the blood vessel layer. The laser beam energy is absorbed when it reaches the blood vessels. The absorbed energy heats the blood and vessels to such high temperatures that they coagulate, or "cook".
After treatment, a normal body function, lymphatic transport, removes the lifeless material. The body then heals, rebuilding the destroyed oversized vessels with normal sized vessels. When the healing process is complete, the properly treated skin has normal blood vessels, permanently removing the PWS birthmark.
The outer skin layer remains intact throughout the laser treatment and protects the underlying tissue. With this normal body defense in place, there is no risk of infection. Because the laser interaction takes place beneath the intact outer skin, the treatment is referred to as sub-epidermal laser surgery.
Prior to the development of the present invention, damage to the dermal and epidermal layers posed a difficult problem in PWS and related laser surgeries. Specifically, if the laser beam is too intense, heat from the coagulating blood vessels "cooks" the adjacent outer skin layers. Once the protective layers are destroyed, the treatment area becomes an open wound, inviting infection. Furthermore, the damaged surface often forms scar tissue on healing. As a result, laser surgery may remove the PWS, but a disfiguring scar replaces the PWS. The patient must then either endure skin graft surgery or live with the scar, depending on medical practicalities.
The skin surface destruction occurs because, prior to the present invention, there was no practical way to predict the necessary laser energy (intensity) required to properly treat PWS. Physicians generally set the laser beam energy, and hence the coagulating heat, based on personal experience. Consequently, some patients receive too much laser energy causing scars, some receive too little laser energy causing no improvement in the PWS condition, and others receive the correct amount and are cured.
One experimental guidance aid uses a Computer Aided Tomographic (CAT) image to improve PWS treatment. However, CAT images are expensive and difficult to employ. Specifically, acquiring the CAT data requires a very expensive infrared imaging camera; processing the image for tomographic display takes several minutes on a premium desktop personal computer; and calculations must be repeated for each 4 square millimeters of treated skin area.
The CAT image provides the physician with a picture of the underlying tissue, but offers no guidance for the desired laser energy settings. Thus, the CAT approach proves both expensive and slow, while leaving the physician to guess at an energy setting based simply on a picture of the underlying tissue.
After considering the prior art systems employed in the laser treatment of PWS and other disorders, it is clear that a convenient and reliable technique is needed by those afflicted with PWS and those treating the disorder. The present invention provides a beam energy control system overcoming the shortcomings of prior art techniques.